In a range of technical applications including the medical field phase contrast imaging has been recognized as superior to solely absorption based imaging techniques for its excellent soft tissue contrast. In some applications phase contrast imaging requires an interferometer mounted in an imaging apparatus. The interferometric set up allows extracting a certain phase shift of interference pattern which has been found to relate to the desired refraction and hence phase shift caused by the object to be imaged. Unfortunately, the phase shift data obtained via the interferometer may be ambiguous. This is because the interferometer has only a limited angular range. Any measurement beyond that range will be “wrapped” around into that range so that in effect it is not the phase shift itself that is observed but only the measured phase shift value “modulo the range”. Phase wrapping is undesirable because it may cause visible artifacts in the phase contrast images. In order to combat this ambiguity a number of “phase un-wrapping” algorithms have been proposed in the past. It has been found however, that the accuracy of the un-wrapped phase data obtainable by existing algorithms is occasionally inaccurate.